With the object of imparting high brightness to light-emitting diodes, the method of relying on the shape of a device for enhancement of the light-extracting efficiency has been in use heretofore. In the device structure that has electrodes each formed on the first surface and the back surface of a semiconductor light-emitting diode, for example, the method for imparting high brightness by the shape of a side face has been proposed (refer to JP-B SHO 63-28508, U.S. Pat. No. 6,229,160 and JP-A HEI 3-227078, for example).
As a Light-Emitting Diode (LED) capable of emitting a visible light in a red, orange, yellow or yellowish green color, compound semiconductor LEDs provided with a light-emitting layer made of aluminum-gallium-indium phosphide ((AlXGa1-X)YIn1-Y P; in which 0≦X≦1 and 0<Y≦1) have been known heretofore. In the LEDs of this kind, the light-emitting part provided with the light-emitting layer that is made of (AlXGa1-X)YIn1-YP; in which 0≦X≦1 and 0<Y≦1) is generally impervious optically to the light emitted from the light-emitting layer and is mechanically formed on the substrate material, such as gallium arsenide (GaAs), which has no appreciable strength.
Recently, for the purpose of obtaining a visible LED exhibiting as high brightness as possible and with the object of further enhancing the mechanical strength of a device as well, therefore, the techniques for configuring a junction-type LED by removing an impervious substrate material, such as GaAs, and thereafter joining by way of innovation a backing layer made of a transparent material capable of transmitting emitted light and excellent in mechanical strength more than ever have been disclosed (refer to Japanese Patent No. 3230638, JP-A HEI 6-302857, JP-A 2002-246640, Japanese Patent No. 2588849 and JP-A SHO 58-34985, for example).
Then, for the purpose of obtaining a visible LED of high brightness, the method that enhances the light-extracting efficiency by dint of the shape of device has been finding acceptance. As one example, in the device structure having electrodes formed each on the first surface and the back surface of a semiconductor light-emitting diode, the technique that relies on the side structure for enhancing high brightness has been disclosed (refer to U.S. Pat. No. 6,229,160 cited above and JP-A SHO 58-34985, for example).
The conventional technique has proposed numerous shapes for devices so configured as to have electrodes formed each on the first surface and the back surface of a light-emitting diode but has not studied the exoergic property exhibited by such a device when the device is used with an electric current of high level. Particularly, the light-emitting diode containing an AlGaInP and gallium nitride-based light-emitting layer provided on a light-extracting surface thereof with two electrodes is inferior in exoergic property to the device structure having electrodes disposed on the back surface because it has no electrode disposed on the back surface. It is known that the deficiency of the exoergic property results in elevating the temperature of the light-emitting layer, lowering the light-emitting efficiency and degrading the brightness.
The structure of the device using a transparent-substrate AlGaInP light-emitting layer and having two electrodes formed on a light-extracting surface entails such problems as complicating the shape, failing to optimize the electrode disposition, the side face condition and the back surfaces of the light-emitting layer and device and not acquiring high brightness and sufficient exoergic property.
While the junction-type LED has enabled provision of an LED of high brightness, the need for an LED exhibiting still higher brightness has persisted. This invention has been proposed in view of the problems mentioned above. In a light-emitting diode possessing two electrodes on a light-extracting surface, this invention is aimed at providing a light-emitting diode that excels in exoergic property, exhibits a high light-extracting efficiency and possesses high brightness.
It is clear even from the conventional technique that the shape of the side face of a light-emitting diode is related to the extraction of light from the diode. In the structure having a light-emitting surface in the upper part, when the angle of inclination is increased for the purpose of rendering the effect of the shape of the side face conspicuous, the increase results in decreasing the area of the back surface, lowering the exoergic property and degrading the property of brightness in the region of high degree of electric current. When the light-emitting layer is made smaller and the area of the back surface is made larger for the purpose of enhancing the exoergic property, the problem of cost ensues because the expensive light-emitting layer incurs a great loss. When the light-emitting layer is approximated to the back surface, the structure that possesses two electrodes on one surface cannot be assembled by the ordinary wire bonding process.
The present inventors, as a result of a comprehensive study pursued on the shape and the back surface of a light-emitting diode, have discovered that the structure and area of the back surface, the area of the light-emitting layer, the shape of the side face and the coarsening of the back surface dictate an important consideration and eventually perfected this invention by finding out the optimum device structure and the stable method of fabrication. Specifically, for the purpose of accomplishing the object described above, this invention has been accomplished.